A four-month experiment was conducted to evaluate the toxicity of a sewage effluent using both single-species and whole ecosystem tests. Fresh effluent from the East Greenwich, Rhode Island, sewage treatment plant was added daily to six experimental ecosystems (mesocosms). Sewage effluent additions were made at 0.1, 1.0, and 10% of the daily seawater input (960 L/day) from Narragansett Bay. Reagent grade, inorganic nutrients were added to the control, 0.1% and 1.0% effluent treatments to normalize nutrient loadings in all treatments. The Arbacia punctulata sea urchin sperm cell test was conducted on effluent and mesocosm samples. In the mesocosms, measurements of responses at the population, community, and ecosystem levels of biological organization were made.

The single-species toxicity test indicated the mean EC₅₀ of the sewage effluent was 1.1%. Toxicity decayed rapidly over time, and was unrelated to carbon, nutrient, residual chlorine, or metal concentrations. Toxicity in the mesocosms was variable due to short-term (4 to 5 h), incomplete mixing of the effluent. There was no evidence for a buildup of toxicity in the mesocosms. Mesocosm effluent additions decreased phytoplankton standing stock and produced an imbalance between total system production and respiration leading to hypoxia. At the 10% effluent loading, net system production was negative.

Results indicated both single-species and mesocosm approaches were useful to assess toxicity. The single-species test was best utilized to characterize the magnitude and persistence of toxicity, and the mesocosm experiments were best employed to identify sensitive communities and processes.